The present invention relates to a melting furnace by electromagnetic induction through the circulation of a high frequency alternating current and more particularly usable for the melting and conversion of refractory materials, such as creamic oxides, glass and chemical salts.
The principle of the induction furnace consists of passing an alternating current through an inductor or induction coil. A magnetic field is then formed within said inductor, in which the charge to be liquefied is located. Induced currents are then generated and flow within the charge, being converted into heat energy by the Joule effect, provided that the resistivity of the charge is below a value dependent on the diameter thereof and the frequency in question.
Numerous refractory materials can be looked upon as insulating at ambient temperature, but the resistivity thereof decreases beyond a so-called inductability temperature. It is in this case necessary to provide a heating means for initiating the induction phenomenon. When the charge has been melted, the furnace can operate in continuous casting manner, provided that the appropriate filling and emptying means are available.
Known devices, such as those protected by French Pat. Nos. 1 430 192 and 1 430 962, together with European Pat. No. 0 079 266, reveal that such melting furnaces can have different design variants.
The inductor may be constituted by a simple conductive metal envelope, which is generally cylindrical and is only interrupted by a slot, to whose terminals the voltage taps are applied. Thus, the current performs a complete turn solely around the charge. This design is called monoturn hereinafter.
The inductor may also be constituted by a solenoid (multiturn design), the current then travelling in a helix.
No matter whether it is of monoturn or multiturn form, the inductor can be insulated from the charge to be liquified by a refractory or cooled wall (indirect induction mode). It can also be in contact with the charge to be liquefied and this represents auto-crucible direct induction. The inductor must then be in principle cooled by a fluid circulation; there then being a solid layer of the refractory material, in pulverulent or granular form, which insulates the inductor from the molten charge.
However, these designs suffer from the following disadvantages. The solutions in which an intermediate wall insulates the inductor from the charge have a reduced efficiency as a result of the Joule effect produced in said wall, as well as the electromagnetic decoupling produced.
The auto-crucible solutions require the positioning of an external envelope in the case of a multiturn inductor in order to prevent the flow of the charge out of the crucible. The monoturn inductor suffers from the disadvantage of the risk of an electric arc forming between the two voltage taps of the inductor, particularly if the outer layer of the charge is raised to a temperature above the inductability temperature. This layer is then no longer able to correctly fulfill its electrical insulation function.
Multiturn inductors suffer from the major disadvantage of their high impedance, the inductance being proportional to the square of the number of turns and to the square of the diameter. It is consequently necesary to use small diameter crucibles (in practice no larger than 35 cm for a winding having two turns), which causes induction problems within the charge and also limits the heat exchange surface between the molten bath and the starting material which is continuously added.
Another disadvantage of monoturns is associated with the risk of an electric arc forming between the voltage taps, as stated hereinbefore. Thus, there is a limitation of the potential differences with which it is possible to work.
The present invention leads to an improvement of existing solutions to the extent that it combines the simplest design, namely the monoturn auto-crucible furnace with a device making it possible to avoid risks of arcs, which constitutes the major problem with the monoturn concept.